1. Field of the Invention
This invention relates to method and apparatus for positioning or pointing a portable earth station antenna system on a satellite in geo-synchronous orbit and more particularly relates to such a system for automatically positioning such an earth station antenna from a remote location.
Recent satellite technology advances have placed satellites utilizing microwave frequency transponders in a geosynchronous orbit. In a geosynchronous orbit, of course, the satellite remains over approximately the same location on the earth surface, twenty-four hours a day and it appears motionless in the sky. The use of extremely high frequency transmitters such as Ku band microwave transmitters in the 14.0-14.5 gigahertz frequency region has provided the ability to reduce the size of parabolic dish antennas used to communicate information from the surface of the earth to the satellite and vice versa. With satellite spacings on the order of 2.degree. apart, along the equatorial plane, it has become more and more difficult to accurately point an antenna at the correct satellite which it is desired to communicate with and to provide such communication without interference to other satellites.
In particular, positioning a small portable antenna by relatively unskilled personnel in field operations, such as in a well logging environment, can require that the specific direction of the radiated signal from the antenna be held within plus or minus one-tenth of a degree of the desired position. To set up an antenna and to send and receive signals from a geostationary satellite in a remote field location is a relatively difficult procedure even when accomplished by highly skilled personnel. The general procedure normally used by such highly skilled personnel would be to point the antenna in the proper direction and to verify that the correct satellite signal is being received by using a spectrum analyzer. Pointing requires the use of an accurate inclinometer and compass.
In the present invention, however, a system is provided for automatically positioning a portable parabolic reflector antenna, which may be located, for example, on the top of a well logging truck, by relatively unskilled personnel and completely automatically under computer control. Before going to a description of the system according to the present invention, however, a brief description of the prior art as known to the applicant is desirable.
2. DESCRIPTION OF THE PRIOR ART
U.S. Pat. No. 3,309,699 to Erdle relates to a mobile tracking system for communication satellites. A beacon signal developed by the satellite is used to broadly point the antenna toward the satellite. Accurate pointing is then achieved by interrogating the amplitude of a return signal generated from a signal transmitted to the satellite by the antenna station. The Erdle patent, however, does not disclose a method for aligning an antenna with a designated satellite by maintaining a constant elevation angle and scanning the azimuth angle automatically to peak on a beacon signal.
U.S. Pat. No. 3,378,845 to Welber discloses a satellite tracking apparatus for continuously directing an antenna toward a satellite. A beacon signal is generated with a particular frequency at an earth station. The satellite reflects the signals back to the earth station where the signal is processed by an antenna drive system (auto track equipment 24). The specific operation of this system is not disclosed in the Welber patent. However, the patent calls for acquiring the satellite by radar equipment or satellite orbital data; see, for example, Column 2, lines 9-15.
U.S. Pat. No. 3,434,142 to Andre, et al and U.S. Pat. No. 3,242,494 to Gicca are also of general interest for their disclosure of antenna position adjusting systems with respect to the present invention. However, neither of these patents disclose a system having the capabilities of the system of the present invention.
3. BRIEF DESCRIPTION OF THE INVENTION
A portable parabolic dish antenna carried on a well logging truck is positioned automatically by a portable computer system driving motors in a remote location according to the concepts of the present invention. Plural servo-motors may be used to drive the portable parabolic antenna in azimuth and elevation. The portable antenna may be, for example, mounted on the roof of a well logging truck in accordance with the concepts of the invention.
The earth latitude and longitude of the well site is entered into the portable truck computer along with the position of the desired satellite which it is to be communicated with on the Ku band. As the geosynchronous satellite will be at a fixed elevation angle relative to a given location, the portable parabolic antenna (whose base has been reasonably leveled) is adjusted by a computer to a predetermined elevation angle and pointed by the computer in a predetermined general direction of the satellite heading in azimuth. While keeping the elevation angle constant, the azimuth angle is swept slowly under computer control. The signal strength of the desired satellite transponder is monitored at the known signal frequency of the transponder. When the satellite signal is detected, a satellite transponder beacon channel is decoded to verify that the desired satellite and the specific transponder is being received. If the beacon signal does not decode, it is assumed that the signals are not from the desired satellite and the scan continued until the desired satellite is found. This azimuthal sweep is performed under computer control with equipment and computer software or firmware as will be described in more detail subsequently.